Within high concentration solar systems we can distinguish the Stirling dishes, cylinder-parabolic collectors and the technology currently at hand, central receiver technology.
Central receiver systems consist of a heliostat field, these being mirrors of large area (40-125 m2 per unit) so-called solar tracking heliostats, which reflect direct solar radiation on one or more central receivers located at the top of a tall tower. These receivers are usually housed in cavities “excavated” in the actual tower.
Concentrated solar radiation heats a fluid in the receiver, thermal energy of which can then be used for generating electricity.
In central receiver systems the water-steam technology is now the more common, as a heat transfer fluid being used both saturated and superheated steam. The stakes are clear for this type of fluid for two reasons: firstly, one can say with fully firmness that steam is one of the fluids more known in this area, with less technological risk. Secondly, it is the final fluid wherewith a turbine works for the generation of electricity, thereby saving in exchange equipment and eliminating the losses associated with them.
Therefore, for such solar thermal power tower plants, there is needed a location which is attached to the existence of two resources: high solar irradiance and sufficient water supply. Generally those areas that meet high levels of irradiance are areas wherein the water resource is limited, which is why plants in the search of more efficient solar receivers, and with the minimum use of water as possible, the present invention is raised, which is intended to recycle and use the highest amount of water with the lowest possible power consumption.
Currently solar concentrator plants for producing electricity work in the following manner: heliostats reflect solar radiation to the receivers that are at the top of the tower, with that energy a fluid evaporates, said steam is sent to a turbine to produce electricity and at the outlet of the turbine the water steam is tried to be recovered, which is still at an elevated temperature. For such reason, they conduct again the water steam leaving the turbine to a condenser. Through this condenser the tap water circulates at a temperature lower than that of the steam, so that the steam gives up its heat to the tap water being condensed and then pumped to recirculate it back to the receiver.
The tap water flowing through the condenser to cool the steam comes out at a temperature higher than that at the inlet.
In order to reuse this water back into the circuit of the condenser, we must lower the temperature thereof. For such reason, there are currently used forced circulation cooling towers by using large fans which allow air circulation and heat exchange between it and the water. These cooling towers are able to diminish the temperature of hot water coming from the condensation circuit by means of heat and mass transfer to the air flowing through the interior of the tower.
In order to improve the air-water contact, a fabric so-called “padding” is used. Water enters into the tower through the top and is evenly distributed over the padding by using sprays. In this way, an optimal contact between the water and the atmospheric air is achieved.
The padding serves to increase the exchange time and surface between the water and the air. Once the contact between water and air is established, a transfer of heat from the water into the air takes place. This takes place due to two mechanisms: the heat transmission by convection and the steam transmission from the water to the air, with the consequent water cooling due to the evaporation that this entails.
In the heat transmission by convection, a heat flow is produced towards the air surrounding the water because of the difference in temperature between the two fluids.
The evaporative cooling rate is of great magnitude in the cooling towers; about 90% is due to the diffusive phenomenon. When the air contacts with water, a thin film of saturated moist air is formed on the water sheet flowing down through the padding. This is because the partial pressure of water steam within the air film is greater than that of the moist air circulating through the tower, resulting in a transfer of water steam (evaporation). This evaporated mass of water extracts the vaporization latent heat from the liquid itself. This latent heat is transferred to the air, resulting in a cooling of the water and an increase of the air temperature.
These aforementioned systems have several drawbacks such as the power self-consumptions generating the use of fans in the cooling towers and the high water consumption required.
These self-consumptions are formed by the assembly of installation equipments which need power consumption for their operation; thereby this consumption must be subtracted from the raw form produced by the installation. If some progress is made towards more reduced self-consumption equipments, there will be also working in the increase of the profitability of the installation.
In order to reduce the power self-consumption in conventional thermal power plants that so-called natural-draught or hyperbolic-draught towers are used.
The flow of air through the natural-draught tower is mostly due to the difference in density between the inlet fresh air and the outlet warm air. The air expelled from the column is lighter than the atmosphere and the draught is created by the chimney effect, thereby eliminating the need of mechanical fans.
The speed difference between the wind flowing at ground level and the wind flowing through the top of the chimney also helps to establish the air flow. For both reasons, the natural-draught towers have to be high, and they must also have a large cross section to facilitate the movement of rising air. These towers have low maintenance costs and are very suitable for cooling large volumes of water. The average speed of air through the tower is usually between 1 and 2 m/s. This type of natural-draught towers do not used very compact “padding”, because the resistance to air flow should be as small as possible.
As mentioned before, these towers are widely used in thermal power plants; wherein despite the construction of the tower supposes a strong investment the creation of this is balanced by lower power consumption.